A USA300 strain of methicillin-resistant Staphylococcus aureus (MRSA) was used in all experiments. Standard techniques were used for RNA isolation, RNA sequencing, and gene expression quantification, neutrophil lysis assays, C3b deposition, and opsonophagocytic assays were performed in vitro. Mouse models of pneumonia and sepsis were employed to assess in vivo virulence.
In the mouse sepsis model (tail vein injection of S. aureus) and in the pneumonia model (intranasal infection), oxacillin given IV at both 7.5 mg/kg and 75 mg/kg increased overall survival. In untreated controls, 8/10 mice had abscesses at necropsy, whereas 4/10 mice treated with low-dose oxacillin and 0/10 mice treated with high-dose oxacillin had abscesses. Also, MRSA cfu/mL were significantly lower in the kidney and spleen in oxacillin-treated mice, with reductions more marked in mice sampled at seven days than in those sampled at 28 hours post infection. In the murine pneumonia model, oxacillin treatment initiated after 3.5 hours increased survival from 20% in untreated mice to 60% in mice treated with either low- or high-dose oxacillin.
In vitro studies demonstrated that oxacillin repressed production of a number of cytolytic toxins. This was mechanistically related to downregulation of accessory gene regulator (Agr).
In experiments in which the USA300 strain was incubated in the presence of subinhibitory concentrations of oxacillin, mRNA expression analysis showed that oxacillin caused down-regulation of 25 genes (including cytolytic toxins and Agr), and up-regulation of 174 genes (including mecA, regulator of toxins-rot, vancomycin resistance regulator-VraR, and genes encoding enzymes involved in cell wall teichoic acid biosynthesis).
Additional in vitro assays showed that oxacillin treatment of USA300 (and two other MRSA strains: JE2 and tarS::Tn) increased C3b deposition and opsonophagocytic killing of these organisms.
COMMENTARY
This paper presents several well-executed in vitro and in vivo experiments, which demonstrate the pleiotropic effects of oxacillin on MRSA. One intriguing randomized, controlled trial of 60 patients with MRSA bacteremia showed that duration of bacteremia was reduced from 3 to 1.9 days in patients treated with both vancomycin and flucloxacillin compared with vancomycin alone.1 The use of β-lactams to repress toxin production, alter cell wall architecture, and increase bacterial susceptibility to host immune cell killing is attractive. Clearly, larger clinical trials will be important to validate the clinical utility of combining β-lactam antibiotics with anti-MRSA agents.
REFERENCE
- Davis JS, Sud A, O’Sullivan MV, et al. Combination of vancomycin and β-lactam therapy for methicillin-resistant Staphylococcus aureus bacteremia: A pilot multicenter randomized controlled trial. Clin Infect Dis 2016;62:173-180.
Using several in vitro assays and animal models, it was shown that oxacillin-treated methicillin-resistant Staphylococcus aureus strains are attenuated in virulence. The effect is mediated by repression of accessory gene regulatory quorum-sensing system and altered cell wall architecture.
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